A watch case responding to this definition has already been described in the patent document CH-338405. In this latter, the watch case includes a caseband, a back cover and a packing inter-posed between the back cover and the caseband. The back cover includes a peripheral flange having an inclined exterior wall in the form of an ellipse or polygon and the caseband includes an annular groove having an inclined wall of a form corresponding to that of the wall of the flange on the back cover. The back cover flange can thus be engaged in the groove of the caseband and be clamped therein by rotating the back cover on the caseband.
This case, however, exhibits the drawback of being subject to rapid wear. Effectively, the elliptic or polygonal form of the peripheral flange and the complementary form of the caseband groove permit clamping of the back cover on the caseband only along the lines of contact which extend in a direction parallel to that of the inclined wall of the flange or of the groove. This has as result that a sealed closing of the case is no longer effectively assured, indeed not at all assured following a certain number of opening and closing operations.
To that is added the fact that the behaviour in compression of the packing is equivalent to that of a seal in a watch case in which the back cover is secured to the caseband by screws and in which the seal is strongly compressed in the region of the screws and very slightly between such regions which leads to sealing problems. Effectively, during tightening of the back cover on the caseband, only the portions of the back cover in contact with the caseband tend to penetrate into the caseband and locally compress the seal in such regions, because of the intrinsic flexibility of the back cover.
The invention thus has as main purpose to overcome the drawbacks of the above-mentioned prior art in providing a sealed watch case comprising a system for securing the back cover onto the caseband having satisfactory and long-lasting effectiveness and which is simple and of easy manufacture.